1. Field of the Invention
This invention relates in general to print output apparatus and methods, and more particularly to a method, apparatus and article of manufacture using media roughness as a print parameter.
2. Description of Related Art
In electronic print devices for printing, coping and desktop publishing systems, images are usually offered in electronic form, and are then referred to as electronic images. These electronic images can be stored permanently on magnetic disk or transported via direct links or networks to the print devices. The creation of a page results in an electronic data stream or electronic file describing the several elements of the page layout in electronic format. This electronic page layout is usually expressed in a page description language. The electronic page layout comprises the data for each electronic image that must appear on the printed reproduction.
In order to render images such as pictures, letters, and characters from input information, halftone screens are used to cause the illusion of continuous-tone pictures when the display or print device is capable of producing only binary picture elements. For example, input images are divided into a number of very small cells (called picture elements herein below) arranged in matrix form and each of the printed images is obtained as a whole by using picture element reproduction elements, one each of which being disposed so as to correspond to a different picture element. In general, according to this method, in most cases, logic circuits such as digital integrated circuits are used in electric circuits driving picture element reproduction elements.
In such a printing device, each electronic gray level, which may be an eight bit number, for example, having a value N between 0 and 255, is converted to a visual density on the hard copy. This density, D, is defined as the negative base ten logarithm of the portion of reflected or transmitted light from incident light on the hard copy. The relation between the gray level N and the density D is strongly dependent on the output device. In order to get the same reproduction from the same electronic page layout on different output devices, the gray level N is not fed directly to the printing device, but rather a stimulus value S is applied to the printing device. The stimulus value S is derived from the gray level N in such a manner that after transformation of the stimulus S to a density D by the printing device, a specific relation exists between the gray level N and the resulting density D. This is called linearization of the printing device, and is thus achieved by establishing a device specific relation between the gray level N and the device stimulus S. This relation is referred to as a tone curve or a transfer function.
It is known in the art that most printing devices have a device specific transfer function incorporated, which is obtained by calibration. Each time when electronic images are printed on this device, the same device specific transfer function is applied to the gray levels of the electronic image.
Problems arise when the output mode of the output device changes without a corresponding change in print parameters. One output device can for example render electronic images on an output medium. The term medium or media indicates the physical material on which the output appears. As applied to paper, “print quality” is an assessment of a particular paper surface respective to the printed product on that surface. One of the more influential objective factors bearing upon the print quality of a paper surface is the “roughness” of that surface. Hereinafter, the terms “roughness” and “smoothness” may be used to refer to the texture of the media surface. Those skilled in the art will recognize that the term “roughness” is merely the opposite perspective from “smoothness”, and that such texture quantisation may be expressed as either with the knowledge that the two properties are inversely related.
A screening algorithm can be used to hide defects in media that the image is printed on. However, the quality of media, for example, how smooth or coarse the media is, can vary. So a screening algorithm that is set up for smooth media may not work as well on course media. But the algorithm for coarse media may remove some image content that could be printed on smooth media.
It can be seen then that there is a need for a selectable halftone screen in a print device, wherein the halftoning algorithm could be changed depending on the roughness of the media being used.
It can also be seen then that there is a need for a method and apparatus that uses pre-defined halftone screens for a range of media smoothness, wherein the pre-defined halftone screens may be stored and selected by the operator or automatically.